Dependence on eluent

Figure 5.1 Pesticides included in the systematic investigations on APCI-MS signal response dependence on eluent flow rate the parameter IsTow represents the distribution coefficient of the pesticide between n-octanol and water. Reprinted from J. Chromatogr, A, 937, Asperger, A., Efer, 1., Koal, T. and Engewald, W., On the signal response of various pesticides in electrospray and atmospheric pressure chemical ionization depending on the flow rate of eluent applied in liquid chromatography-mass spectrometry , 65-72, Copyright (2001), with permission from Elsevier Science.

ELSD Quasi-universal No dependence on eluent conditions Droplet size control Moderate sensitivity (low ng) Compound-dependent and non-linear detector response [31,51-53]... 

Although most classical silica-ODS columns are not stable at temperatures much above 60°C (dependent on eluent conditions), columns made from other materials such as polymeric and zirconia-based phases have been used for high temperature work (see earlier text). 

APCI is the simultaneous formation of different adduct ions. Depending on eluent composition and matrix components, it is possible that Na" " and NH4 adducts can occur besides protonated analyte molecules, making the data evaluation more difficult. 

FIGURE 6.8 Guidelines for Shodex column selection depending on the molecular weight of polymer. Column Shodex GPC KF-800 series, 8 mm i.d. x 300 mm. Eluent THE. Flow rate 1.0 mUmin. Detector Shodex Rl. Column temp. 40°C. Sample EPIKOTE 828... 

Using other methods for the calculation of plate count can result in different numbers, depending on peak shape. It should also be kept in mind that many other operational parameters, such as eluent viscosity, column temperature, flow rate, and injection volume, will influence the results of the plate count determination. 

The plate number for the same column also depends on the eluent, e.g., a permitted operation for some styrene-divinylbenzene columns is to change the eluent from tetrahydrofuran (THF) to dimethylacetamide (DMAC) and then return to THF. The plate number in DMAC is considerably lower than in THF. After the replacement of DMAC by THF the old N value is obtained again. 

Giddings pointed out (32) that separated compounds must remain resolved throughout the whole process. This situation is illustrated in Figure 1.5, where two secondary columns are coupled to a primary column, and each secondary column is fed a fraction of duration Ar from the eluent from the first column. The peak capacity of the coupled system then depends on the plate number of each individual separation and on At. The primary column eliminates sample components that would otherwise interfere with the resolution of the components of interest in the secondary columns. An efficient primary separation may be wasted, however, if At is greater than the average peak width produced by the primary column, because of the recombination of resolved peaks after transfer into a secondary column. As At increases, the system approaches that of a tandem arrangement, and the resolution gained in one column may be nullified by the elution order in a subsequent column. 

Assuming that the adsorbent surface is occupied by an adsorbed solute molecule (Ag) and a molecule of a stronger solvent (n ) which are equal to one another, the elution strength of a binary eluent, shows the following dependence on its quantitative composition ... 

Figure 6.27c, this procedure is illustrated by showing the band of the mixture throughout the whole width of the chromatographic plate [26,27]. In the next stage, the development of the chromatogram proceeds. It is performed by the introduction of an appropriate developing solvent into the eluent reservoir. It is recommended that at hrst a small quantity of the eluent be introduced into the reservoir to wash the reservoir walls and eliminate remnants of the sample solution. An example of separation of the sample mixture using this procedure is shown in Figure 6.28. Even a few milliliters of the sample solution can be applied with this mode. However, it depends on the sample mixture.

The optimization of preparative and even micropreparative chromatography depends on the choice of an appropriate chromatographic system (adsorbent and eluent), sample application and development mode to ensure high purity, and yield of desirable compounds isolated from the layer. For the so-called difficult separations, it is necessary to perform rechromatography by using a system with a different selectivity. But it should be taken into account that achievement of satisfactory results frequently depends on a compromise between yield and the purity of the mixture component that is being isolated. 

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